Code:
ServerAlias webmail.*
Code:
Alias /webmail /etc/sentora/panel/etc/apps/webmail
This allows all domains and subdomains to use domain.com/webmail to access roundcube and has worked without flaw for a few years now.
(I also use a similar variation for external phpmyadmin login.)
Doing it this way also prevents clients from having to add a webmail record to their DNS settings. Much easier for everybody